With the advent of microlithography methods and apparatus employing a charged-particle beam (CPB), a chronic difficulty has been achieving high exposure resolution and high throughput. Various approaches have been proposed to such end.
A first approach, tantalizingly offering prospects of high throughput, is an apparatus that exposes at least one entire die per exposure. One disadvantage of this approach is that it is difficult to manufacture a CPB-exposure mask from which an entire die can be projection-exposed in one shot. Another disadvantage is that it is exceedingly difficult to satisfactorily control aberrations over an entire die that is exposed in one shot. In fact, the goal of aberration control has been elusive with this first approach.
A second approach, offering prospects of improved resolution, utilizes "segmented exposure" in which a mask corresponding to an entire die is divided into multiple "subfields" that are individually exposed step-by-step. Thus, multiple shots are required to projection-expose a corresponding die on a sensitive substrate ("wafer"). Because the CPB is much smaller in transverse area using this second approach, the prospects for satisfactory aberration control (especially in peripheral regions of each shot) are more favorable.
An electron-optical theory applicable to certain types of segmented exposure is disclosed in Koikari et al., "Numerical Calculation On Optical System for EB Projection," Proceedings of the 9th International Microprocessing Conference ("Microprocess '96"), Jul. 8-11, 1996. Koikari et al. pertains to an electron-beam (EB) optical system employing a symmetric magnetic doublet lens system comprising first and second static axisymmetric lenses. The Koikari et al. CPB optical system also comprises multiple magnetic deflectors that impart paraxial deflections to an electron beam propagating, from a point on the mask, through the EB optical system to the wafer. The deflections purportedly reduce certain (but not all) aberrations. Also, the Koikari et al. deflectors impart no net deflection of the electron beam at the wafer plane. I.e., when electrons in the electron beam reach the wafer plane, all deflections imparted by the deflectors during propagation of the electrons from the mask to the wafer are exactly cancelled.
Consequently, the Koikari et al. theory, as presented in the Koikari et al. paper, can be applied only to segmented exposure in which individual mask subfields are contiguous (i.e., not separated from each other by boundary regions occupied by struts or the like).
Deflectors according to Koikari et al. are typically energized by respective power supplies under the control of a "controller" (i.e., numerical processor or computer). Each signal sent by the controller to a respective power supply is a function of the X- and Y-coordinates (X.sub.m, Y.sub.m) of the illuminated mask subfield but does not take into account the X- and Y-coordinates (X.sub.w, Y.sub.w) of the corresponding transfer subfield on the wafer.
FIG. 3 is a plan view of a wafer 11 exposed using a conventional electron-beam exposure system exhibiting aberration correction in a manner such as disclosed in Koikari et al. The optical axis AX of the system extends perpendicularly to the plane of the page at the position indicated. FIG. 3 also shows the relationship between the position of illuminated mask subfields 100a, 100b (relative to the optical axis AX) and the positions of the corresponding transfer subfields 200a, 200b (relative to the optical axis AX).
A CPB exposure apparatus embodying the principles of Koikari et al. has "zero deflection sensitivity" with respect to the deflectors, by which is meant that the deflectors collectively impart no net deflection of the electron beam from a wafer position that would be illuminated if no aberration-correcting deflectors were provided (or if the aberration-correcting deflectors were not energized. Also, according to conventional wisdom, the central coordinates (X.sub.w, Y.sub.w) of a transfer subfield are related to the central coordinates (X.sub.m, Y.sub.m) of the corresponding mask subfield by the following equations (1) and (2): EQU X.sub.w =-M(X.sub.m .multidot.cos .theta.-Y.sub.m .multidot.sin .theta.)(1) EQU Y.sub.w =-M(X.sub.m .multidot.sin .theta.+Y.sub.m .multidot.cos .theta.)(2)
wherein M is the combined magnification (M&gt;0) of the static axisymmetric projection lenses and .theta. is a rotational angle of the image of the mask subfield formed on the respective transfer subfield.
To increase the mechanical rigidity of a segmented mask and render the mask more resistant to thermal distortion during illumination of the mask by the charged-particle beam, it is preferable to provide the mask with struts. Typically, the struts are placed between mask subfields. When the mask subfields on such a mask are projected onto the wafer, the struts are not projected. In order to form a complete integral image of the die on the wafer, all the transfer subfields of the die must be properly "stitched together." Thus, even though the mask subfields are separated from one another on the mask by the struts, the corresponding transfer subfields of the complete die formed on the wafer must be "connected" together without any intervening gaps. The charged-particle beam used to make such an exposure must be deflected in a manner ensuring that strut regions are not projected and that the transfer subfields in each die are properly stitched together on the wafer.
In order to stitch the transfer subfields together on the wafer when using a segmented mask with struts, aberrations must be controlled very well to ensure accurate alignment of all the transfer subfields with each other. Such aberrational control includes control of "hybrid" image-plane distortions and blur. (A "hybrid" distortion at a given point in an image is a function of both the distance of the point from the optical axis and the distance of the point from the center of the subfield. See Zhu et al., "Dynamic Correction of Aberrations in Focusing and Deflection Systems with Shaped Beams," SPIE 2522:66-77.) Functions according to, e.g., Koikari et al. are simply inadequate for use with a segmented mask including struts. Koikari et al. does not address how to make appropriate changes to the functions. Also, changing functions and/or adding more deflectors, without more, can undesirably cause aberrations to increase.
Therefore, there is a need for charged-particle-beam exposure apparatus that: (1) utilize a segmented mask including struts separating the mask subfields, (2) stitch together the corresponding transfer subfields on the wafer, and (3) satisfactorily reduce third-order aberrations.